Add email sync service for automated job application tracking

Background poller fetches emails via IMAP or Microsoft Graph API,
classifies them with Claude Haiku, and creates/updates JobApplication
records automatically. Includes manual sync endpoint and OAuth callback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/go.sum

@@ -66,6 +66,12 @@ github.com/dgryski/trifles v0.0.0-20230903005119-f50d829f2e54 h1:SG7nF6SRlWhcT7c
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+github.com/emersion/go-imap/v2 v2.0.0-beta.8 h1:5IXZK1E33DyeP526320J3RS7eFlCYGFgtbrfapqDPug=
+github.com/emersion/go-imap/v2 v2.0.0-beta.8/go.mod h1:dhoFe2Q0PwLrMD7oZw8ODuaD0vLYPe5uj2wcOMnvh48=
+github.com/emersion/go-message v0.18.2 h1:rl55SQdjd9oJcIoQNhubD2Acs1E6IzlZISRTK7x/Lpg=
+github.com/emersion/go-message v0.18.2/go.mod h1:XpJyL70LwRvq2a8rVbHXikPgKj8+aI0kGdHlg16ibYA=
+github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6 h1:oP4q0fw+fOSWn3DfFi4EXdT+B+gTtzx8GC9xsc26Znk=
+github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6/go.mod h1:iL2twTeMvZnrg54ZoPDNfJaJaqy0xIQFuBdrLsmspwQ=
 github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.1-0.20191026205805-5f8ba28d4473/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.4/go.mod h1:6rpuAdCZL397s3pYoYcLgu1mIlRU8Am5FuJP05cCM98=